Hello there Rosamaria. Maybe I can help answer your question.
In Genesis 2:18 God says "It is not good that man should be alone; I will make him a helper comparable to him" (NKJV). Literally the verse should read "it is not good for the man to be alone" however the above quote is how it is translated in the NKJV. The NASB, ESV, and other translations do a better job than the NKJV does with this verse. So the verse is really talking about "the man" or Adam. It does not refer to every man or every person.
Now let's look at the context of the verse. God created animals and created two of each kind of animal (Gen 2:19-20). So every animal had another like it and, because they were opposite sexes, every animal could produce others like it (Gen 1:24-25). However Adam was alone. As we see in Genesis 2 Adam was created without a partner so he was literally "alone". He was the only human on the planet. Therefore God, on the sixth day, created Eve so that Adam would not be the only human on the planet. God called this good (Gen 1:31). When He says in Genesis 2:18 that "it is not good that man should be alone" He is refering to this situation. It is not good for Adam to be all alone on a big planet. God created people for fellowship with Himself but also for fellowship with other people.
The situation in Genesis 2:18 does not exist today. In our day even if a person is not married they are not "alone". They have family, friends, if they are a Christian they have their church family, the people they work with, etc. People today are not alone therefore Genesis 2:18 does not deal with the issue of singleness and marriage today.
As for 1Corinthians 7, well, I think Paul's statements are pretty clear. Paul states that it is good not to get married if a person can remain single and pure. However if they cannot then they should marry (1Cor 7:6-9, 25-28,32-38).
Hope that helps!
LOVE 1 Cor 7 
